1. Field of the Invention:
The present invention relates to porous materials useful for separating and concentrating gases having a molecular weight not larger than 4, such as hydrogen and helium from a mixture of gases containing such gases, and a process for separating and concentrating such gases using such porous materials. More particularly, this invention relates to porous materials having more than 50% of fine pores falling in a specific range of pore diameter, and a process for separating the above gases in a high concentration with a high efficiency using such porous materials.
Since the oil shock in recent years, use of petroleum oil has been discouraged in most iron works, and many blast furnaces have been operated on a coke-straight base, where heavy oil blowing has been replaced by increased use of coke. As a result, so-called "iron works gases" which are given off in the course of making iron and steel in iron works increase in amount. The iron works gases have hitherto been recovered for reuse in the works, but under the recent situation, new uses of such gases must be developed.
The iron works gases include coke oven gas, converter gas and blast furnace gas. Hydrogen is a main constituent of a coke oven gas, while carbon monoxide is a useful main constituent of the converter and blast furnace gases. Carbon monoxide may be readily converted by a known process, Schift reaction, into hydrogen and carbon dioxide. Therefore, as one way of using the iron works gases for applications other than fuels, it may be concluded that if hydrogen in iron works gases or hydrogen and carbon deoxide converted by the Schift reaction can be economically separated and concentrated, the use of iron works gases can be extended into other fields such as for raw material of organic chemical syntheses. In a wider scope, separation of a particular constituent of a gas has practical importance. For instance, attention has been paid to separation of hydrogen from an off-gas exhausted from ammonia synthesis or petrochemical works and separation of helium from natural gas.
Methods for separating hydrogen or helium from mixed gases in a high concentration using porous materials or porous membranes are conventionally known. These methods utilize diffusion of hydrogen or helium molecules through tiny pores of the porous materials for separation of hydrogen or helium. This is based on the well known theory that the flow rate of a gas molecule through pores is proportional to the molecular velocity, so long as .lambda./d&gt;&gt;1, where .lambda. is the mean free path of the gas molecule and d is the pore diameter. More particularly, the flow rate of a gas through pores is reciprocally proportional to the square root of the molecular weight of the gas molecules in question. Since molecular weights of hydrogen and helium are markedly smaller than those of other gas molecules, the former gases can be separated from other gases merely by diffusion through pores. Feasibility of separation by diffusion requires the ratio .lambda./d to be much larger than unity, that is in other words the pore diameter of the porous materials is much smaller than the mean free path of the gas molecules involved. To this end, a porous material having remarkably small pore diameter is needed. However, even if an ideal separation efficiency can be obtained by providing very small pore diameters, the rate of gas through the pores will be too small and the efficiency be too low for a practical separation apparatus for a commercial application. On the other hand, the mean free path of a molecule can be increased by elevating the temperature and lowering the pressure of the gas to obtain a larger value of .lambda./d. In this case, even an ideal separation efficiency may be obtained, the rate of gas flow through the pores is inpractically small. In fact, the separation efficiency and the permeability through pores have been generally recognized to be incompatible to each other.
So far, many investigations have been made on the separation of hydrogen by means of a porous membrane. However, an industrial apparatus for separation of practical significance has not been realized, because no separation method which is excellent in both separation efficiency and permeability has been established.
It is a general knowledge that, for separation of gases, better permeability is obtained by porous membranes than by non-porous membranes. A separation method which utilizes the above-mentioned characteristics of porous membranes and which shows excellent separation efficiency and permeability, if found, would make it possible to provide an efficient method to separate a high concentration of hydrogen or helium at a lower cost on a commercial scale from iron works gases, off-gases from chemical industries, natural gases and other unutilized gases.
2. Description of the Prior Art:
Japanese Laid-Open patent application No. Sho 55-119420, discloses use of a porous material having 20 to 200 .ANG. pore diameters for separating concentrated hydrogen from a mixture gas containing hydrogen.
This prior art, however, teaches nothing more than a generic range of pore diameter smaller than the mean free path of hydrogen or helium molecules which can be presumed from the theory of gas separation by means of the porous material.
Thus, the prior art relates to a process for separating hydrogen for recovery from hydrogen sulfide decomposition products by using porous hollow glass or porous ceramic fibers, where a range of pore diameter of the porous materials is disclosed to be not smaller than 20 .ANG. from the permeability of hydrogen molecules and not larger than 200 .ANG. from the standpoint of separation from the undecomposed gases. The disclosure, however, refers only to the well-known range of pore diameter in a generic expression, and no particular technical thought which aims at a remarkable effect by using a narrow, specific range of pore diameter is observed. In fact, a porous membrane of mean pore diameter 48 .ANG. is used in the example of the prior art, but the pore diameters are expressed only by the mean value and no description is given to the pore diameter distribution.
In addition, for separation and concentration of hydrogen, not small a number of investigations have been made by using organic high molecular porous membranes as porous materials. For example, D. F. Bradley and R. W. Baker (Polymer Engineering and Science, July 1971, Vol. 11, No. 4) estimated permeability coefficients of gases for high polymer membranes having a mean pore diameter from 20 to 120 .ANG., according to which the flow of the gases is the Knudsen flow for the mean pore diameter 20 .ANG. and the Poiseuille flow for the mean pore diameter 120 .ANG.. These results do not coincide with those obtained with the porous materials of this invention. The difference is due to difference in the nature of pores of organic high molecular thin membrane from those of inorganic material and also to difference in the methods of estimating the pore diameter distribution.
In other prior known methods of separating gases by use of porous materials, pore diameters are only disclosed in a generic expression within the scope of the gas separation theory.